My friend I sold my v6 mustang to last year texted me today asking if i would put the stock air intake back on since I put on the CAI a few months before i sold it. I asked him why and he said it won't accelerate past 40 mph and he went to autozone and thier code reader thing said it was running lean. He apparently has had the fuel pump replaced I put in new spark plugs and plug wires about a year and a half ago. The egr was replaced and fuel filter and maybe a couple other things. When i sold it to him there was a little stall when you would get on it when your on the highway not an exacty mph but somewhere about 50 maybe lower i never thought it was that big of a problem but now apparently it is. What can I tell him to try.

(how much do you think the car is worth 2001 v6 auto probably almost 150,000 miles and the body is in the same shape as it was when i sold it? i really want it back)

Back to the original problem. It sounds to me like clogged catalytic converters to me. That's if it's running crappy too. If it runs fine but won't go any faster, then it COULD be a trans problem and unrelated to the code, but my money is on the Cats. See if any of them are getting really really hot. I've seen them so clogged that they glow orange after a short drive.